(1) Field of the Invention
The present invention relates to a rotational position-detecting device which is capable of detecting the rotational position of various types of rotating shafts in an accurate and simplified manner.
(2) Description of the Related Art
In general, the control of the rotational position of various types of rotating shafts, which is performed e.g. for remote directional control of an antenna, is carried out by detecting the rotational position of a rotating shaft and thereby stopping the rotating shaft in a specified rotational position. There are strong demands for low-cost implementation of such rotational position control of rotating shafts.
A particular rotational position of a rotating shaft can be detected by an optical detecting element or a magnetic detecting element arranged at a location corresponding to the particular rotational position of the rotating shaft. Therefore, it is possible to detect each of a plurality of rotational positions of the rotating shaft by providing a plurality of detecting elements of an above-mentioned type around the rotating shaft at locations each spaced by a predetermined interval of an angle of rotation and corresponding to the respective rotational positions of the rotating shaft. However, to detect the rotational position more finely or at reduced intervals of the angle of rotation, it is required to provide a very large number of detecting elements, which makes a device employing this rotational position detecting method impractical.
Therefore, conventional rotational position-detecting devices generally use an encoder provided on a rotating shaft. The encoder comprises a disc mounted on the rotating shaft, which is formed with through holes spaced at precisely identical circumferential intervals, and a light emitting block and a light receiving block with the disc interposed therebetween. The light emitting block emits a light ray toward the light receiving block, while the light receiving block receives the light ray transmitted thereto from the light emitting block via each of the through holes of the disc, and delivers a pulse signal in response to the incident light ray. Pulse signals thus produced are counted starting from one corresponding to a predetermined reference rotational position of the rotating shaft. This makes it possible to detect the rotational position of the rotating shaft by resolution corresponding to an interval between adjacent ones of the through holes of the disc.
To enhance resolution in the detection of the rotational position of the rotating shaft, however, the conventional encoder is required to have a disc formed with a large number of through holes spaced at precisely identical intervals, which is disadvantageous in that the manufacturing cost of such a disc is very high.
Further, in remote control of a rotating shaft, an output signal from the encoder is required to be transmitted to a distant location. If the rotational speed of the rotating shaft is not uniform, the output signal from the encoder contains pulses occurring at largely-varying intervals. The transmission of such a pulse signal requires a signal transmitting system which is complicated in construction. Further, if remote control of a device or a system should be performed by controlling the rotational positions of a plurality of rotating shafts used therein, an encoder is required to be provided for each of the rotating shafts, and output signals from the encoders must be transmitted to a distant location, which is disadvantageous in that the whole remote control system becomes even more complicated and expensive.
Further, to detect a particular rotational position of the rotational shaft, it is necessary to know the number of pulses generated during rotation of the rotating shaft from the aforementioned reference position to the particular rotational position to be detected, and hence it is required to constantly store results of detection performed on the preceding occasion, in order to detect the rotational position of the rotating shaft in a continuous manner.